1. Field of the Invention
This invention relates to a method for drawing a mother ingot into a preform for optical fibers wherein when the mother ingot is drawn into preforms for optical fibers, a finishing step can be omitted or simplified.
2. Description of the Prior Art
For the fabrication of preforms for optical fibers form synthetic silica, a soot prepared, for example, by VAD is dehydrated and sintered to provide a mother ingot for preform. This ingot, usually having a diameter of 110 mm to 200 mm, is drawn to a diameter, which is larger by 3 to 5 mm than the diameter of the preform (e.g. 30 to 80 mm in diameter). In this drawing step, a glass-working lathe making use of an electric furnace or a flame burner has been usually employed. If the ingot has an outer diameter as small as 100 mm or below, it is possible to use a glass-working lathe for preliminary or primary drawing. For ingots having larger diameters, a flame burner is short of the quantity of heat, and thus, an electric furnace is ordinarily used. An electric furnace, which is employed for the drawing of an ingot, has such a structure that an ingot is fed form an upper portion of the electric furnace heated at about 2000.degree. C., and a diameter-reduced preform is continuously taken off by means of rollers or drawing chucks from the lower portion of the furnace. This is particularly shown in FIG. 5, which illustrates the manner of the take-off of the preform by use of drawing chucks. In addition, outer diameter measuring instruments are provided to measure outer diameters at a neck portion of the ingot and also at a preform, respectively.
FIG. 6 is a flowchart showing a conventional process of drawing a preform. In the figure, a mother ingot is furnace-drawn such as in an electric furnace, and thus, is reduced in diameter to provide a primary-processed product, i.e. a preform. The outer diameter intended upon the drawing is set at a value greater by 5 to 10% than final intended value. The preform is subjected to measurement of a refracting index profile or distribution in radial directions. Based on the results of the measurement, optical transmission characteristics after drawing to a final optical fiber are estimated. The characteristics are decided depending on the refractive index profile or distribution of the preform, and especially, a ratio in diameter between the core portion and the clad portion (hereinafter referred to simply as "core-to-clad ratio") is an important parameter. In order to change the core-to-clad ratio, it is usually to etch the outer surface of the clad portion with hydrofluoric acid. By the etching, the core-to-clad ratio increases to an extent which is in coincidence with intended characteristics. More particularly, the characteristic properties of the ingot, which may vary from predetermined ones by many influences involved during the fabrication of a mother ingot, can be corrected according to subsequent processing steps. From the results of assumption of the characteristics, preforms whose characteristics should be corrected are etched for the correction. Those preforms, which need no correction, are fed to a next finishing step.
The finishing step is normally performed by means of a glass-working lathe. The primary drawing of the ingot in an electric furnace is carried out within a relatively short time of 1 to 2 hours. In this connection, however, the final drawing with a glass-working lathe takes about 1.5 hours per unit preform when a perform having a length of 1000 mm and a diameter of 43 mm is adjusted, for example, to a diameter of 40 mm. Thus, it takes an appreciably long time for processing preforms one by one, and it is desirable to reduce the degree or amount of the final drawing to an extent as small as possible.
In the above-stated conventional method, the index distribution is measured in the form of primary-processed preforms to assume optical transmission characteristics, and each preform is etched based on the results of the measurement as described hereinabove. This, in turn, requires that all preforms including ones, which do not need any adjustment of the characteristics after drawing, have to be drawn at a diameter, which is larger by 5 to 10% than an intended diameter. This means that preforms, which are not required of etching, have to be unavoidably subjected to final drawing.